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Izzie returns to work amid fears of impending layoffs, which lead residents to jockey for positions at the hospital. Meanwhile, Bailey and Alex's difficult case is further complicated by the patient's schizophrenic son. Jodie: Adrienne Barbeau. Olivia: Sarah Utterback.

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It's Day 1 of the Seattle Grace-Mercy West merger, and residents react as though their jobs are on the line. They are, especially Izzie's. Meanwhile, Callie's father (Hector Elizondo) gets a priest to urge her to date men. Callie's enraged, and Arizona stands by her.

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Arizona faces a conflict of interest over a 10-year-old patient whose rich parents are planning to donate $25 million to the hospital. Meanwhile, Cristina tests her boundaries with Owen at the hospital; and Alex must deal with Izzie's hospital bills.

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Owen recruits a fellow Iraq-war vet (Kim Raver) as the new cardiothoracic surgeon, and Cristina questions the move for both personal and professional reasons. Meanwhile, Izzie returns with her high-school mentor, who's seeking treatment for dementia; and the stresses caused by the merger are taking their toll on the Chief. Dr. Singer: Joel Grey. Adele: Loretta Devine.

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Bailey's father (Frankie Faison) visits for the holidays, and he voices his disapproval of her life choices. Meanwhile, a woman from Mark's past shows up, complicating things with Lexie; and Meredith defends the Chief when he's criticized by Thatcher (Jeff Perry). Teddy Altman: Kim Raver.

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Addison (Kate Walsh) arrives from L.A. to help Mark with a difficult operation on his pregnant daughter (Leven Rambin); Owen questions Teddy's motives when she assigns Cristina to lead a complicated surgery; and the Chief recruits Meredith to assist with a high-profile operation, raising Derek's suspicions. Ruthie: Cynthia Stevenson.

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Derek begins his new role. Meanwhile, Bailey and Meredith's patient awakens from anesthesia mid-surgery; Teddy refuses to place Cristina on her service while Arizona tests Alex in pediatrics; and Mark refuses to speak to a heartbroken Lexie.

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A roof collapses on a restaurant known as a trysting spot on Valentine's Day, interrupting whatever romance plans the doctors might have had. Nonetheless, Arizona and Callie play Cupid---for Bailey and an anesthesiologist named Ben (Jason George). Callie also teams up with Mark to help persuade Sloan (Leven Rambin) to do the right thing for her baby. Meanwhile, Derek struggles to manage his time in his new role as chief, and Meredith wrestles with what being the chief's wife entails.

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Richard, Bailey and Callie recall important cases in their careers in a lecture series that new chief Derek reintroduces. For Richard, it's an AIDS case (before it was called AIDS) in which he collaborated with young Ellis Grey (Sarah Paulson); Callie looks back on a polio case; and Bailey recalls her early residency---and her rivalry with another resident (Missi Pyle).

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Tensions mount between Derek and Richard (and the staff is on edge) when a famous surgeon with a tie to one of the doctors is rushed to the hospital and requests an unorthodox procedure. Meanwhile, Callie and Arizona try to help Teddy and Sloan get over past disappointments.

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Owen recalls wartime experiences and his past with Teddy after she asks him for help with a patient who wants to die; Callie and Arizona have differing visions of their future; and Richard adjusts to his new role as a surgeon. Kim: Sara Gilbert.

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Sloan (Leven Rambin) returns just as she's about to give birth, and Mark must deal with the prospect of his grandson's adoption. Meanwhile, Teddy feels threatened when Derek makes overtures to a distinguished cardiothoracic surgeon; and the captain of a crab boat is impaled by a shark hook.

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Bailey conducts a mandatory sensitivity-training session before a 700-pound patient arrives with numerous medical problems that prove challenging. And those aren't the case's only challenges. Meanwhile, Derek must encounter a late patient's husband in a wrongful-death deposition; and spending time with a heart patient's daughter opens old wounds for Cristina.

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An elderly heart patient (Marion Ross) meets an long-lost lover in the ER, prompting the charmed staffers to reassess their own romantic lives. Meanwhile, Alex treats a troubled teen (Demi Lovato) whose parents say she is schizophrenic; and Meredith expresses her suspicions about Owen to Cristina.

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Part 1 of 2, Season 6 finale. Meredith has news for Derek, but tells Cristina first. Then, the husband of patient (Michael O'Neill) who died during surgery arrives at the hospital with a gun and looking for Derek. Mary: Mandy Moore.

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Conclusion, Season 6 finale. Shooter Gary Clark (Michael O'Neill) remains on the loose, presenting Cristina and Meredith with major surgical challenges. Meanwhile, Bailey and Mary (Mandy Moore) must find a way to get the injured Charles to an ER.